Production of pulp, etc.



atented Aug. 9, 1932 LINN BRADLEY, OF MONTCLAIR, NEW JERSEY, AN!) EDWARD1?. MCKEEFE, OF PLATTS- BURG, NEW YORK, ASSIGNORS TO BRADLEY-MCKEEFECORPORATION, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK I PRODUCTIONOF PULP, ETC.

1W0 Drawing.

This invention relates to improvements in the production of pulp andtreatment of residual liquors of the digestive operations for theregeneration of cooking liquors.

More particularly the invention relates tov cyclic interrelatedprocesses, employing three types of cooking liquors which may becharacterized briefly as sodium sulfite type, sodium sulfid type andcaustic soda type cooking liquors, and in which residual liquor from asodium sulfite cooking process is treated to produce a cooking liquorfor a sodium sulfid cooking process, and in which residual liquor from asodium sulfid cooking process is treated to produce a cooking liquor fora caustic soda cooking process, and in which residual liquor from acaustic soda cooking process may be treated to produce a sodium sulfitecooking liquor. In these cyclic and inter related operations some of thesame soda is employed in one type of cooking liquor and then in another.Soda is employed as a sulfite of sodium in a sodium sulfite cookingprocess, then in part as sodium sulfid in a sodium sulfid cookingprocess, and thereafter in a caustic soda cooking process.

According to one embodiment of this invention, a sodium sulfite type ofcooking process is carried out with a cooking liquor contain ng sodiumsulfite, either alone or admixed with certain other ingredients.Residual liquor therefrom is concentrated and subjected to a reducingfurnace treatment to convert some of the sodium-sulfur compounds intosodium sulfid, with the remainder of the soda largely present as sodiumcarbonate. A. cooking liquor is produced, from the resulting product,which contains considerable sodium sulfid, and may contain otherreagents, such as caustic sodaor sodium sulfite, or both, etc. Thiscooking liquor is employed for a digestion treatment. Residual liquorfrom the latter cooking'operation, in which the sodium sulfid containingliquor is employed, is treated to produce a cooking liquor whichcontains caustic soda as the primary active const'tuent and may beeither substantially free from sodium sulfid or contain sodium sulfid inrelatively smaller amount. This caustic soda liquor is employedApplication filed September 2, 1926. Serial No. 133,290.

for a digestive treatment. The residualliquor from the caustic sodacooking operation may be treated to produce a sulfite of sodium type ofcooking liquor. Soda is thus employed as a sulfite of sodium in thesulfite type of cooking process, then in part as sodium sulfid in thesulfid type of cooking process, and next largely as caustic soda, andthen again in some cases as a sulfite of sodium. Sulfur is employed as asulfite of sodium, next as sodium sulfid, and the residue is thenlargely reoxidized or removed, and may be recovered for further use, e.g. in the sulfite type of process or in the sulfid type of process, orin part in each of these types.

The sulfite of sodium type of cooking process may be an acid, 'a neutralor an alkaline sodium sulfite cooking process. That is, the cookingliquor may contain sodium monosulfite alone, or with smaller amounts ofcertain other ingredients; or it may be an alkaline liquor containingsodium sulfid or caustic soda or both, as well as sodium sulfite; or itmay be an acid liquor containing. sodium bisulfite, or a mixture ofsodium monosulfite and" sodium bisulfite, or even sulfur dioxide beyondthe bisulfite stage, i. e. containing NaI-TSO- and H 50 Cooking liquorscontaming only or mainly sodium monosulfite, or sodium monosulfitetogether with a lesser amount of'sodium thio-sulfa'te, or with sodiumsulfid or with caustic soda, etc., may be used, for example, cookingliquors such as de scribed in our prior applications Ser. Nos. 481,147,filed June 28, 1921, 686,137, filed J anuary 14, 1924, 711,857 filed May8, 1924:, or 704,176 filed April 4, 1924. Or the cooking liquor may bean alkaline cooking liquor conprocess, in which the cooking liquorcontains primarily caustic soda and a lesser amount of sodium sulfid asthe active constituents; or it may be a modified process differing fromthe ordinary sulfate process cooking liquor by containing sodium sulfidas the primary cooking chemical and being made up e. g. mainly of sodiumsulfid and sodium carbonate; or it may contain also some caustic soda orsome sodium sulfite or various mixtures of these compounds. Cookingliquors such as described in our prior application Ser. No.

57,954 filed September 22, 1925, may be used,

containing caustic soda and sodium sulfid together with sodium sulfite.The cooking liquor of the sodium sulfid cooking process need not containsodium sulfid as the main constituent, but it nevertheless generallycontains it in fairly large amounts.

The caustic soda type of cooking liquor, which is the third type of theseries of cooking liquors, may be the ordinary soda process cookingliquor or a modified caustic soda liquor, or it may be an ordinarysulfate process cooking liquor, but sodium sulfid, if present therein,will be generally in smaller amounts than in the cooking liqgor of thesodium sulfid process, and it maiy be absent therefrom. The causticsodatyp of cooking liquormay contain some sodium sulfite, but in generalit will be made up mainly of caustic soda, i. e., it will containcaustic soda as.

the predominant active constituent.

The residual liquor from the sulfite type of cooking process, especiallywhen neutral or alkaline, may be directly and readily concentrated. Ifacid, it may be advantageously neutralized prior to concentration, bythe addition thereto of alkaline residual liquor or of recoveredalkalinecompounds from one or another of the regenerative processes. Afterneutralization or having been made alkaline, the liquor may then be moreadvantageously concentrated, thus retaining sulfur more readily. Theconcentrated residual liquors, or the solids contained therein afterdrying, are subjected to a reducing furnace treatment which decomposesorganic co1npounds and converts sodium-sulfur compounds to aconsiderable extent into sodium sulfid, so that the product of thereducing furnace treatment contains the sodium compounds mainly assodium carbonate and as sodium sulfid, generally with a considerable.

amount of sodium sulfid. So'me sulfur may be driven off (e. g. ascombustible sulfur compounds) during this reducing heat treatment whichcan he recovered, e. g. from the furnace gases after oxidizing them,recovering sulfur trioxide and sulfur dioxide, e. g? for use in thesulfid and in the sulfite type of cooking liquors, respectively.

The furnace product containing the sodium compounds mainly as sodiumcarbonate and as sodium sulfid may be dissolved to form a solution whichmay, in some special cases, be used directly as acooking liquor, inwhich case the cooking liquor will consist mainly of sodium sulfid andsodium carbonate. In this case the sodium present as sodium sulfid andas sodium carbonate may advantageously be approximately equal. Insteadof uslng the solution directly as cooking liquor, some of the sodiumcarbonate can be causticized with lime to form caustic soda, or sulfitede. g. by treatment with calcium sulfite, or in part causticized and inpart sulfited, to give a cooking liquor containing sodium sulfid andcans. tic soda, or sodium sulfid and sodium sulfite, or sodium sulfid,caustic soda and sodium sulfite. The cooking liquors as employed shouldhave the required activity for the result which is desired.

Residual liquor from such a cooking operation in which sodium sulfid ispresent, is concentrated and subjected to a furnacing treatment, whichmay be either reducing or oxidizing, If the furnacing operation isreducing,

some sodium sulfid may be present in the furnace product. If thefurnacing operation is oxidizing,the furnace product may be free fromsulfid and may contain some sodium sulfate, or sodium sulfite, or both.Such reducing furnace product usually contains a higher amount of sodiumcarbonate and a lower amount of sodium sulfid than in the case of thereducing treatment of the sulfite type of residual liquor above referredto. In such case, by highly causticizing a solution of the recoveredsodium compounds a relatively small or moderate amount of sodium sulfidmay be present and accompanied by a large amount of caustic soda. Whenthe furnacing operation is carried out under oxidizing conditions, thefurnace product also will contain the sodium compounds largely as sodiumcarbonate, and, by highlycausticizing a solution thereof, a cookingliquor can he produced consisting mainly of caustic soda. Some of thesoda can be sulfited where sodium sulfite is desired in the caustic sodatype of cooking liquor. For example, one-part may be causticized',another part'sulfited, and these admixed in proper ratio.

Residual liquor from the caustic soda type of cooking operation may beconcentrated and organic matter decomposed. Organic compounds may becarbonized or burned preferably under oxidizing conditions, so that anysulfur residue thereof may be in part removed and in part oxidized tosulfate or sulfite. If surfur compounds are absent from such causticsoda cooking liquor, oxidation or removal of sulfur is not required. There sult of such heat treatment of residual liquor from the caustic sodatype of digestion is to yield a furnace product in which sodiumcarbonate is the main sodium compound present, and from whichsodium'sulfid is absent or is present therein in only small amount.solu- .tion of the sodium carbonate so produced can be readily sulfitede. g. with S0 gas to regenerate the sulfite of sodium type of cookingliquor. If this cooking liquor is to be an alkaline sulfite liquor, or aliquor containing mainly sodium monosulfite, some thiosulfate may alsobe present. If the cooking liquor is to be a highly acid sulfitesolution, e. g. with a large excess of sulfur dioxide beyond the sodiumbisulfite stage, the sodium carbonate solution, which is to be sulfited,is preferably free from sodium sulfid and thiosulfate.

Instead of operating the same number of digesters with each of the threetypes of cooking liquors, or instead of using all of the residual liquorfrom each type of cooking operation to produce cooking liquor for thenext type of digestion of the cycle, part of the residual liquor fromone type of cooking operati on may be treated to regenerate similarcooking liquor for the same type of cooking operation, or residualliquors from difierent types of cooking operations may be admixed inregulated proportions and the admixed liquors then treated to regenerateone or another of the three types of cooking liquors. For example, inmaking cooking liquor for the sulfid-containing cooking operation, someresidual liquor from the sulfid containing cooking operation may beemployed for neutralizing an acid sulfite residual liquor, or foradmixture with a neutral or an alkaline sulfite residual liquor, and theresulting admixed liquors may thenbe treated in the manner described forproducing a sulfide-containing cooking liquor. The cycle hasconsiderable flexibility, so far as the number of digesters cooked orrelative amounts of pulp produced by the respective types of cookingoperations are concerned.

The supply of soda and of sulfur to make up for losses may take place atVarious suitable points in the cycle. Where sodium sulfate is available,it may be added with advantage to the sulfite type residual liquor, orthe dried product thereof, so that sodium sulfid will be producedtherefrom duringthe reducing furnace treatment, thus supplying sodiumand some sulfur to the cycle. Sulfur may be supplied in-the form ofsulfur dioxide by burning sulfur in air or by roasting iron sulfide orother sulfides. Sodium carbonate may be supplied and may be sulfited toform a sulfite of sodium, or be causticized to form caustic soda.Caustic soda, sodium sulfid, etc. may be supplied at any suitable pointin the cycle.

A considerable amount of the sulfur which may be driven off in the formof oxidized compounds or of combustible sulfur compounds can berecovered for further use. Combustible gases from the reducing furnacetreatment of sulfur-containing residual liquors can thus be oxidized orburned to convert combustible sulfur compounds, when present, largely into sulfur dioxide or trioxide, or both, which can be recovered byscrubbing the gases, e. g. with residual liquor i from. In case sulfuris oxidized to sulfur trioxide, it can be recovered e. g. by'scrubbingthe gases with black liquor or with a sodium carbonate solution, assodium sulfate, and the sodium sulfate can be returned to the cycle andutilized for supplying sodium sulfid to the sulfid type of cookingliquor.

As an example of a sodium sulfite type of cooking liquor which can beused in the sulfite type of cooking operation of the invention, we maymention a suitable volume of cooking liquor containing sodiummonosulfite to the extent of about 35 to 40 per cent on the oven-dryweight of the wood, in the case of poplar wood, or to the exent of 40 to50 per cent on the oven-dry weight of the wood, in the case of sprucewood.

As an example of cooking liquors containing both sodium monosulfite andsodium bisulfite, a cooking liquor may be used containing 1200 pounds ofsodium monosulfite, and about 500 pounds of sodium bisulfite to 4000pounds of wood (oven-dried basis), in the case of poplar wood, or-about1335 pounds of sodium monosulfite and about 550 pounds of sodiumbisulfite to 4000 pounds of wood (oven-dried bases), in the case ofspruce wood. Such liquors may also contain a relatively small amount ofsodium thiosulfate and also of sodium sulfate. The sodium-sulfurcompounds in the resulting residual liquor form some sodium sulfid forthe sulfid type of cooking liquor, during the reducing furnacetreatment.

As examples of an acid sulfite cooking liquor, beyond the bisulfitestage, the liquor may contain 990 pounds of sodium bisulfite and about260 pounds of sulfurous acid (H SO to 4000 pounds of wood (oven-driedbasis) in the case of poplar wood, or about 1240 pounds of sodiumbisulfite and about 325 pounds of sulfurous acid (H SOQ in the case ofspruce wood. Sodium sulfate may also be contained in such liquor.

A. liquor containing about 1320 pounds of sodium besulfite to 4000pounds of wood (oven-dried basis) in the case of poplar wood, or about1650 pounds of sodium bisulfite to 4000 pounds of spruce wood(oven-dried basis) can be used and sodium sulfate may also be present insuch liquors, e. g., to the extent of 200 or 300 pounds.

The sulfite type of liquor when alkaline may also contain some sodiumcarbonate, caustic soda or sodium sulfid. As an example of a liquorcontaining a large amount of sodium sulfite, and also containingconsidersulfite and about 640 pounds of sodium sulfid,

together with about 870 pounds of sodium carbonate to 4000 pounds ofpoplar wood (oven-dried basis). In the case of spruce wood we may useabout 227 5 pounds of sodium monosulfite and about 700 pounds of sodiumsulfid to 4000 pounds of wood (oven-dried basis). The cooking operationin such cases can be carried out, for example, at a temperaturecorresponding to a saturated steam pres sure of 130-150 pounds persquare inch and for a sufficient time to complete the cooking operation.

As examples of cooking liquors, relatively high in sodium sulfid, we mayuse, for example, a cooking liquor containing about 800 pounds of sodiumsulfid and about 1090 pounds of sodium carbonate to 4000 pounds ofpoplar wood; or about 1000 pounds of sodium sulfid and about 1360 poundsof sodium carbonate to 4000 pounds of spruce wood. Or we may preferablyuse a cooking liquor also containing sodium .sulfite; for example,liquor such as above described con taining in addition to the sodiumsulfid and sodium carbonate, about 320 pounds of sodium sulfite forpoplar wood, and about 400 pounds of'sodium sulfite for spruce wood.

As examples of cooking liquors containing caustic soda, as well assodium sulfid in rela tively large amount, we may use a cookingliquorcontaining about 600 pounds of sodium sulfid and about 800 pounds ofsodium carbonate, together with about 200 pounds of caustic soda to 4000pounds of poplar wood, or about 750 pounds of sodium sulfid, about 1000pounds of sodium carbonate, and about 250 pounds of caustic soda to 4000pounds of spruce wood; or about 250 pounds of NaOH, 750 pounds Na S, 400pounds Na sO 375 pounds of Na GO or about 300 pounds NaOH, 600 pounds ofNa s, 500 poundsof Na SO 275 pounds of Na CO or 400 pounds of NaOH, 600pounds Na s, 400 ounds Na sO and 200 pounds of Na 0 for poplar or sprucewood.

Such cooking liquors can be produced by treating the solution containingmainly sodium carbonate and a relatively high sodium sulfid content,with a regulated amount of lime to form the required amount of causticsoda from the sodium carbonate, and with calcium sulfite sufficient toform the required sodium sulfite from some of the sodium carbonate, or aproperly proportioned mixture of lime and calcium 'sulfitc may beemployed, Where the cooking liquor is to contain both caustic soda andcalcium sulfite, as well as sodium sulfid.

As an example of'a cooking liquor relatively high in causticsoda, we mayuse a cooking liquor containing about 640 pounds of caustic soda, andabout 375 pounds of sodium sulfid to 4000 pounds'of pine wood(oven-dried basis). Such a liquor contains a considerable amount ofsodium sulfid,'and illustrates a cooking liquor which may be producedfrom the furnace product obtained by treating sulfur-containing residualliquors under reducing conditions, or a cooking liquor which may beproduced, e. g. from the sulfite type of residual liquor when theoperation is carried out so that a relatively large amount of sodiumcarbonate is produced and a smaller amount of sodium sulfid, and bysuitably causticizing' the sodium carbonate. By admixing difierentproportions of the sulfite type of residual liquor and of the sulfidtype of residual liquor, and subjecting the admixed products to reducingfurnacing treatment un'der regulated and suitable conditions, the ratioof carbonate to sulfid in the furnace product can be somewhat controlledand therefrom a caustic soda cooking liquor can be produced of varyingsodium hydroxide and. sodium sulfid content. Example: 800 pounds NaOI-I,150 pounds Na S, 200 pounds Na CO for 4000 pounds of wood (oven-driedbasis).- These cooking liquors may contain some sodium sulfite.

Wherc thecaustic soda cooking liquor is produced from the furnaceproduct formed from the residual liquor of the sulfid type of cookingprocess, especially when carrying out the furnacing operation underoxidizing conditions, it may be free, or nearly so, from sodium sulfid.Example: 1000 pounds NaOH, 200 pounds Na SO 200 pounds Na COa for thesame amount of wood.

It will thus beseen that the present invention includes cookingoperations in which cooking liquors of three different types areemployed; that residual liquor from the sulfite type serves to provide arecovered soda product containing a rather large portion of its sodiumcontent as sodium sulfid, which is employed in a cooking liquor of thesodium sulfid type; while residual liquor from the sulfid type of,cooking operation serves to provide a recovered soda product forpreparing a cooking liquor of the caustic soda type, which is employedin a caustic soda cooking operation; and residual liquor from the caus-1 tie soda type of cooking operation provides a recovered soda productwhich is readily utilizable by sulfiting to form a cooking liquor, ofthe sulfite type, which may be employed to produce pulp by a sulfitetype of cooking operation, and to form a. residual liquor which can beadvantageously utilized in the further carrying on of the cycle. Sodamay be thus employed in a cyclic manner, while sulfur may also beemployed in a cyclic manner, serving as a sulfite, then as sulfid, thenbeing in part removed or oxidized or both. Some sulfur may be recoveredfor reuse, e. g. as a sulfite, or as sulfate, or'both.

The residual liquor from the caustic soda type of cooking operation maybe treated in any suitable manner. It may be evaporated to dryness andthe solids subjected to a heat treatment under oxidizing conditions toyield a furnace product containing mainly sodium carbonate. Most of thesulfur contained in such residual liquor either is removed by the gasesor converted into an oxidized compound by such treatment and thusconserved or fixed. Therefore some sodium sulfate may be present in thefurnace product along with the sodium carbonate. This sodium sulfate maythus be present in the sulfite type of cooking liquor which. may bereadily prepared by dissolving the sodium compounds,

clarifying as needed, and sulfiting the solution, e. g. by S gas. .Whenresidual liquor from the sulfite type of cooking operation is subjectedto the reducing furnace treatment, this sodium sulfate is useful informing some sodium sulfid for the sulfid type of cooking.

liquor. The sulfur which is fixed as sulfate is thus'conserved. Asstated, Na s is advantageously absent from the Na CO solution Which'isto be sulfited, especially when the sulfite type of cooking liquor is tobe of a highly acid character, since Na S O whlch might otherwise beformed, is detrimental in such highly acid liquors and firthermoreincreases the sulfur requirement Instead of carbonizing or burning theorganic compounds in the residual liquor com ing directly from thecaustic soda type of cooking operation, this residual liquor may besulfited directely by SO gas to decompose some of the compounds thereinand form a sulfite of sodium liquor. Some of the organic compounds arethereby decomposed and some organic matter is precipitated. Thecompletely sulfited or the partially sulfited liquor may be separatedfrom the precipitate in any suitable manner, and the separated liquor,after further sulfiting if needed, be employed for cooking wood with thesulfite type of cooking liquor thus prepared, all as more particularlyset forth in our U. S. Patent No. 1606501.

This residual liquor from the caustic soda type of cooking operationmaybe employed to scrub furnace gases which contain oxidized sulfurcompounds, e. g. S0 Such liquor may be further sulfited, if necessary,

e. g, after separation from precipitated or- 'anic matter, in order toproduce a cooking liquor containing the desired amounts andratio ofcombined to uncombined S0 Tnasmuch as such employment of this residualliquor for the-scrubbing operation may result in. a liquor whichcontains some sodium thiosulfate, the sulfite type of cooking liquorthus prepared should have sufiicien't" com;- bined S0 to overcome oravoid the objectionable effects of Na2S203, such as those when it ispresent in considerable amount in sulfite type of cooking liquors whichcontain furnace treatment. Such sodium organic compounds as remain inthe cooking liquor thus prepared, i. e. by direct sulfiting of suchresidual liquor with S0 are present in the sulfite type of cookingoperation andserve to form some Na CO in the subsequent furnacetreatment.

The following will serve to illustrate, in a. general way the employmentof the three types of coo ring liquors in a cyclic manner which has someadvantages.

A solution of sodium carbonate is sulfited by means of S0 gas, which maybe obtained by burning sulfur in any suitable manner, to form a solutionwhich contains principally sodium monosulfite. This solution is employedin suitable amount for the cooking of wood chips and may contain aroundto parts of S0 (total) to 100 parts of Wood (oven-dry basis). The"digester may be gradually and slowly heated to the cooking temperature,e. g. around 165 to 180 deg. (l, and the cooking liquor is adequatelycirculated and maintained at a suitable temperature until the cookinghas been carried to the proper degree, when the digester is blown in thecustomary manner. When such a cooking liquor is employed, little if anyS0 is released from the liquor, differing from the usual acid sulfitetype of cooking operation in which calcium bisulfite is employed. Bythus retaining the S0 in the liquor, special S0 recovery means are notnecessary, and the cooking liquors can be readily prepared of amplestrength and at a fairly high temperature thus facilitating thepreparation of the cooking liquor and conserving heat units.

The residual liquor is separated from the pulp, and may then beneutralized or made moderately alkaline to litmus by the addit1onthereto of some residual liquor from the sulfid type of cookingoperation, or by some ofthe solution of the recovered soda compounds ofthe furnace product obtained by the subsequent furnace treatment. Theneutrztllzed or alkaline residual liquor is then concentrated and thesolids subjected to a re and sodium sulfid, the operation being soregulated that a considera le amount of sul- .uct contains principallysodium carbonatefur is retained as sodium sulfid in the soda product,thus avoiding the removal of an excessive amount of sulfur along withthe furnace gases, e. g. as sulfur or as an oxidizable sulfur compound.The soda in the resulting furnace product is dissolved in water and thesolution is causticized by suflicient lime to convert a large part ofthe sodium carbonate into sodium hydroxide, thus yielding a cookingliquor containing principally sodium hydroxide and sodium sulfid. Thesodium sulfid content thereof will generally be quite high, e. g. around30 to 50 parts Na s for each to 50 parts NaOH. If desired, a moderateamount of Na SO may be produced in or added to the cooking liquor. Forexample, by using some CaSO along with the Ca(OH) or by sulfiting aportion of the soda solution prior to causticizing, or withoutcausticizing the sulfited portion and with causticizing of the otherportion of the soda solution and mixing the sulfited portion with thecausticized portion. After separating the lime mud from thecookingliquor, the cooking operation iscarried out therewith, this beinga sulfid type of cooking operation. The residual liquor from suchcooking operation is separated from the pulp, is concentrated in Whole,or a portion thereof may be diverted for admixing with the residualliquor from the sulfite type of cooking operation as described above.The solids in the concentrated portion of the residual liquor aresubjected to a furnace treatment, e. g. of a reducing type, so as toproduce a furnace product containin mainly sodium carbonate, although amo erate amount of sodium sulfid may with advantage be present therein.In such treatment, a considerable amount of sulfur is removed along withthe furnace gases.

Sodium compounds in the resulting furnace product, principally sodiumcarbonate, are dissolved in water, and the solution is causticized bytreatment with a' regulated amount of lime to convert nearly all of thecontained sodium carbonate into sodium hydroxide. If desired some sodiumsulfite may also be present in this cooking liquor, but generally theamount thereof will be small. This cooking liquor is the caustic sodatype of cooking liquor. Where the solution of therecovered sodacompounds contains some Na s, the cooking liquor thus prepared ma have asmall} or moderate amount of Na but the priifcipal active ingredientinthe. caustic soda type of cooking liquor issodium hydroxide,considerable of the original sulfur content having been eliminated ordisplaced from its combination or association with soda. This highlycaustic liquor is employed as a cooking liquor, being especiallysuitable for producin soft stock, e. g. pulp from deciduous woocfs, suchas poplars, etc.

The cooking operation with the highly convert most of the soda intosodium carbonate and to eliminate some of the sulfur residue and tooxidize such sulfur as remains with the soda. The sodium compounds aredissolved in water, the solution clarified, if required, and then may besulfited to produce a cooking liquor which contains, for example,

mainly sodium sulfite and sodium bisulfite, although the cooking liquormay contain some sodium sulfate, as stated. And'the cycle may then berepeated.

Instead of employing sodium sulfate for replacing lost soda, sodiumcarbonate may be employed for this purpose, and it may be added at anysuitable point, e. g. before causticizing to prepare the highly causticliquor, or prior to sulfiting to prepare the sulfite type of cookingliquor.

Some of the sulfur .accompanying the furnace gases may be recovered inthe manner stated or in any other suitable manner, and the recoveredsulfur be returned to thecycle.

Where the furnace gases carry both S0 and S0 the S0 content may belargely absorbed in one liquor and the S0 content largely in anotherliquor, and the various compounds thus obtained can be returned to thecycle at their most advantageous points, respectively. By removing mostof the SO, from the gases, e. g. by electrical preci itation, or byabsorption in black liquor rom the sulfid type of cooking operation,which is to be reduced'to form a furnace product containing Na CO and NaS, the S0 remainl ing in the gases is more readily available forsulfiting soda to form a sulfite of sodium for use in the sulfite typeof cooking liquor. A solution of Na CO or black liquor from the causticsoda type of cooking, may be used 1 to absorb SO from the gases. Thusthe recovery of the sulfur content is made more feasible and in aparticularly advantageous and poplar and other deciduous woods may becooked with the caustic soda type of cooking liquors to produce sodapulp.

' The sulfur employed in the sulfite type of cooking liquors isutilized, at least in part, for preparing Na s. The w s thus preared ispresent in cooking liquor employed or digesting wood and the reactionbetween sodium monosulfite,

the wood substance and the soda facilitates the subsequent production ofa recovered soda product which contains a relatively smaller amount ofNa S, and finally the residue of sulfur is in part separated from thesoda and may be in part converted into an oxidized compound, being thusconserved and made again available in part, as Na S upon a'furtherreduction treatment.

Instead of employing a mixture of sodium disulfite and sodiummonosulfite in the sulfite type of cooking liquor, the liquor may befree from sodium bisulfite, or it may even be a mixture of sodiumbisulfite and sulfurous acid. Sodium bisulfite solutions, free from andfrom excess sulfurous acid, may be employed, if desired.

Where a relatively large amount of the alkaline types of pulps aredesired, i. e. produced by the sulfid type and caustic soda type ofcooking liquors, and a relatively small amount of pulp by the sulfitetype of cooking liquors is desired, just enough of the soda from thefurnace product resulting from the furnace treatment of residual liquorfrom the caustic soda type of cooking operation may beodiverted andsulfited to produce the required amount of cooking liquor of the sulfitetype, the remainder of the soda from such furnace product beingcausticized for either the sulfid type or the caustic soda type. ofcooking liquor, as conditions may justify. Such sulfur dioxide as isemployed in producing the cooking liquor of the sulfite type is utilizedas a sulfite for cooking wood an subsequently in the formation of someNa s. The employment of the soda successively in the two types ofcooking operations, viz.: The sulfid type and the caustic soda type, isadvantageous in that a soda product is thus 7 obtained which is readilysulfited by SO to produce the sulfite type of cooking liquor. Na SO isadvantageously employed for replacing lost soda as it provides somesulfur as well as soda. This is especially advantageous when S0 and S0are reclaimed from gases of the furnace operations.

Where the sulfite type of cooking liquor is prepared by directlysulfiting residual liquor from ation, the separated organic matter mayadvantageously be added to the material which is to be subjected to areducing furnace treatment to form Na S for the sulfid type of cookingliquor.

In case a relatively large amount of pulp is to be produced by thesulfid type of cooking operation, a regulated portion of the blackliquor from such cooking operation may be treated to produce a furnaceproduct relatively high in sodium carbonate and containing only amoderate amount of sodium sulfid, by employing a reducing furnacetreatment and adding no additional sodium-sulfur compounds thereto, orthe furnace product may the caustic soda type of" cooking oper- P befree -from sodium sulfid and contain primarily sodium carbonate withperhaps some sodium sulfate where the furnace operation is of anoxidizing type. The soda compounds are dissolved and the solution highlycausticized toproduce a moderate amount of cooking liquor of the causticsoda type. Such cooking liquor is employed to produce pulp and theresidual liquor thereby formed is treated to produce a cooking liquor ofthe sulfite type and the latter is employed for producing pulp and aresidual liquor which is then utilized for producing additional amountsof sodium sulfid, e. g. by admixing With the portion of black liquorfrom the sulfid type of cooking operation which remains after the otherportion has been diverted. Such residual liquor from the sulfite type ofcooking operation should be admixed quite uniformly with all of thisremaining portion of the black liquor to insure the production of acooking liquor for the sulfid type of cooking operation, which is fairlyuniform in its ratio of NaOH to Na S. Sodium sulfate sufficient to makeup for soda losses may be employed for such purpose, e. g. by adding itin a uniform manner to the material which is to be subjected to thereducing furnace treatment for producing the Na S for the suldadvantage, as well as the sulfur.

Where the sulfite type of cooking liquor is to have a high acid content,e. g. of NaHSO and H2803, and in case this type of cooking liquor is tobe prepared by directly sulfiting, by means of S0 residual liquorresulting from the caustic soda type'of cooking operation, the causticsoda type of cooking liquor free from Na S; however, Na SO may bepresent therein. \Vhen sufiicient S0 has been thus dissolved'in such aresidual liquor to precipltate substantially all of the precipitatibleorganic matter therefrom, it is advisable to separate the solution fromthe susended matter and to thereafter continue the sulfiting until theliquor contains the desired ratio of combined to uncombined SO Since theliquor still contains some ganic matter which may be capable of reactingwith sulfite, it is advisable to have the cooking liquor contain moretotal S0 per unit of wood to be cooked than'if the cooking liquor werefree from organic compounds when charged. An increase of, for example,10 to 15% in the total SO over the usual practice will generally besuflicient for the purpose. When such highly acid liquors are employed,the cooking operation can be carried out in the usual acid sulfitecooking manner, relieving S0 and utia mixture.

it is preferable to have dissolved orcooking operation, sodium lized togood advantage;

lizing it in any suitable Way, e. g. for fortifying the raw acid liquorand thus raising the content of uncombined S0 therein.

Instead of preparing the sulfite type of freshly supplied for replacinglost soda may be sulfited in solution to prepare the sulfite carbonateand free from large amounts of sotype of cooking liquor. from thecaustic soda type tion may in such instances be utilized in any othersuitable manner, e. g., for regenerating a cooking liquor containingcaustic soda, either accompanied by or free from Na s. perating in thismanner, the sulfur and the sodium carbonate freshly supplied, are firstemployed in a sulfite type of cooking operation and the residual liquorthereby formed is utilized for the production of some sodium carbonateand some sodium sulfid. A cooking liquor of the sulfid type is thenprepared and utilized for a cooking operation and the resulting blackliquor is employed, at least in part, for the production of a cookingliquor containing a relatively high content of sodium hydroxide, whichis employed in a cooking operation of the caustic so atype.

It will thus be seen pable of numerous variations- The black liquor ofcooking operafeatures will be apparent to those skilled in the art.

Cellulosic materials other than Wood may be pulped by one or more of thethree types of cooking operations described herein.

Ve claim:

ing conditions to ing sodium sulfid, producinga cooking liquorcontaining such sodium sulfid and cooking wood therewith, treatingresidual liquor from such cooking operation to form a difa cookingliquor.

he process of producing pulp which comprises cooking Wood with acookingliquor containing sodium hydroxide and sodium sulfid, treating theconstituents of the resulting residual liquor by a reducing furnacetreatment to form a product containing sodium carbonate and a relativelysmall amount of sodium sulfid, dissolving the soda compounds and tionwith lime treating the soluby a preponderating sodium carbonate isconcooking liquor for the cooking of Wood. producing pulp from of thethree liquors,

first as sulfite, then as sulfide and then as hydroxide, and utilizin aportion of the sulfur content repeatedly,

first as sulfite and then as sulfide.

In testimony whereof we affix our signatures.

LINN BRADLEY. EDWARD P.

